Upgrading to FluentValidation 8

FluentValidation 8.0 is a major release that included several breaking changes. Please review this document before upgrading from FluentValidation 7.x to 8.

Asynchronous Validation updates

There have been many underlying changes to the asynchronous validation workflow in FluentValidation 8.


If you make heavy use of asynchronous validation in FluentValidation, we would really appreciate you trying out the preview builds of FluentValidation 8 and report any issues that you encounter.

SetCollectionValidator is deprecated

Instead of using SetCollectionValidator you should use FluentValidation’s RuleForEach support instead:

FluentValidation 7:

RuleFor(x => x.AddressLines).SetCollectionValidator(new AddressLineValidator()); 

FluentValidation 8:

RuleForEach(x => x.AddressLines).SetValidator(new AddressLineValidator());

Why was this done?

SetCollectionValidator was added to FluentValidation in its initial versions to provide a way to use a child validator against each element in a collection. RuleForEach was added later and provides a more comprehensive way of validating collections (as you can define in-line rules with RuleForEach too). It doesn’t make sense to provide 2 ways to do the same thing.

Several properties have been removed from PropertyValidator

CustomStateProvider, Severity, ErrorMessageSource and ErrorCodeSource are no longer directly exposed on PropertyValidator, you should now access them via the Options property on PropertyValidator instead.

Why was this done?

It allows extra options/configuration to be added to property validators without introducing breaking changes to the interface going forward.

ValidatorAttribute and AttributedValidatorFactory have been moved to a separate package

Use of the ValidatorAttribute to wire up validators is no longer recommended and have been moved to a separate FluentValidation.Attributes package.

  • In ASP.NET Core projects, you should use dependency injection the service provider instead (this has been the default behaviour)
  • For desktop or mobile applications, we recommend using an IoC container to wire up validators, although you can still use the attribute approach by explicitly installing the FluentValidation.Attributes package.
  • In legacy ASP.NET projects (MVC 5 and WebApi 2), the ValidatorAttribute is still the default approach, and the FluentValidation.Attributes package will be automatically installed. However, we recommend using an IoC container instead if you can.

Many old and deprecated methods have been removed

FluentValidation 8 removes many old/deprecated methods that have been marked as obsolete for a long time.

  • Removed the pre-7 way of performing custom validation (Custom and CustomAsync). Use RuleFor(x => x).Custom() instead. See the section on Custom Validators
  • The old localization mechanism that was deprecated with the release of FluentValidation 7. This included several overloads of WithLocalizedName and WithLocalizedMessage. See the section on localization for more details.
  • The RemoveRule, ReplaceRule and ClearRules methods that have been marked obsolete for many years (FluentValidation does not offer a replacement for these as runtime modification of validation rules is not recommended or supported in any way)
  • Removed various async method overloads that didn’t accept a CancellationToken (use the overloads that do accept them instead.)

Other changes

IStringSource.GetString now receives a context, instead of a model. If you have custom IStringSource implementations, you will need to update them.